Lithium in staurolite and its petrologic significance

Abstract

Natural metapelitic staurolites contain appreciable amounts of lithium. Lithium contents were determined by ion microprobe with concentrations of representative samples independently analyzed by atomic absorption spectrophotometry for calibration. Seventy-one percent of the analyzed staurolites contain >0.1 wt.% Li₂O, although the distribution is skewed to values less than 0.3 wt.%.

High Li contents observed in staurolite are attributed to one or more of several factors: initiation of staurolite breakdown, lack of additional host phases for lithium (e.g. biotite), pre-metamorphic Li-rich bulk rock composition, and/or interaction of the rock with Li-rich fluids. Li content is generally not correlated with the modal amount of staurolite in the rock, rather Li values tend to reflect variable host rock Li. Lithium most likely resides in the R²⁺ tetrahedral site. Its incorporation into the structure is probably related to a coupled substitution with Al: ^ivLi ^viA_1/3 ^ivR ²⁺₋₁ ^vi□_−1/3 When staurolite analyses yield low R²⁺ and high Al values, the possibility of high Li should be considered after accounting for variable H.

Lithium partitions into common pelitic metamorphic minerals in the order staurolite>cordierite>biotite>muscovite> garnet, tourmaline, and chloritoid. Partitioning is non-ideal in staurolite and a function of Fe content. Li in staurolite expands its stability field to a higher T relative to garnet and sillimanite, and to a lower T relative to chloritoid and Al-silicate. Analysis of staurolites for Li may provide further insight into this enigmatic mineral.